Our aim is to study the organization of neurotransmitter receptors on nerve and muscle cells in relationship to the development and function of synapses. We have used alpha-bungarotoxin as a specific probe for the visualization and quantitation of nicotinic acetylcholine receptor sites. Our recent work has focused upon the factors, extrinsic and intrinsic to the developing skeletal muscle fiber, which regulate the distribution of nicotinic acetylcholine receptors. We have now provided further evidence for a role of a basement membrane glycoprotein, laminin, in modulating the receptor aggregation induced by neuronal factors. This modulation appears to depend on the binding of laminin to the muscle cell surface. We have also shown that receptor aggregation can be rapidly induced by neuronal factors, with correlated changes in cytoskeletal structures beneath the cell membrane